Coherent summation across a phased array approximates the diffraction integral across a lens. Consequently, coherent summation can be used to spatially localize objects in a field. Incoherent summation across an array, however, does not spatially localize objects, but reduces the variance in scattering measurements since each group of array output signals represents an estimate of the scattering function obtained from a different (i.e., independent) view. Consequently, coherent processing of array data affects spatial resolution and incoherent processing affects contrast resolution.
Large-aperture phased arrays can be used to increase the spatial resolution in images produced by vibrational energy, such as ultrasound. However, coherence cannot be maintained over large apertures in many imaging applications in medicine, resulting in less than optimal spatial resolution.